Conventionally, a quartz type film thickness detector is used in a film thickness measurement of an adhered/deposited substance to be evaporated in a vacuum evaporation apparatus, a plasma CVD apparatus, a sputtering apparatus, and the like. The quartz type film thickness detector utilizes a phenomenon that the resonance frequency of a quartz oscillator changes according to the thickness of an evaporated film, and has features such as high detection precision and a high response speed.
In recent years, an electron impact excitation spectroscopy, which utilizes excited light upon electron impact of an evaporating metal, tends to be used in such a film thickness measurement. The film thickness measurement based on the electron impact excitation spectroscopy utilizes the following phenomenon. When a thermoelectron is caused to collide against the vapor flow of an evaporating substance to excite the evaporating substance, light having a wavelength spectrum inherent to the substance is emitted, and the light intensity at that time is proportional to the density of the vapor flow, i.e., the evaporation rate. In particular, this film thickness measurement has a feature that the film thickness of a two-element simultaneous evaporated film can be detected by providing an optical filter to a detection unit.
As another prior art, an amorphous semiconductor thin film having a large Seebeck coefficient, which can be utilized as a temperature sensor or a strain sensor, has been reported (U.S. Pat. No. 4,766,008, and Published Unexamined Japanese Patent Application No. 62-47177).
As a conventional method of detecting thermal conductivity, a method of measuring the thermal conductivity of a substance by bringing a thermal conductivity detection element into contact with the substance to be measured is adopted (Published Unexamined Japanese Patent Application No. 49-70672). More specifically, the thermal conductivity detection element is brought into tight contact with a substance to be measured, and a cyclic current is flowed through the thermal conductivity detection element to cause endothermic and exothermic reactions in a surface of the thermal conductivity detection element, which surface is in tight contact with the substance to be measured. At the same time, the temperature of the surface of the thermal conductivity detection element, which surface is in tight contact with the substance to be measured, is measured to detect a delay of a phase or a difference in amplitude ratio of the measured temperature waveform with respect to the current waveform flowed through the thermal conductivity detection element due to the thermal physical property of the substance to be measured. Thus, the thermal conductivity of the substance to be measured is measured using the density and specific heat values of the substance to be measured, which are obtained beforehand by some method. This method has a merit that a measurement apparatus is relatively light in weight.
As another thermal conductivity measurement method, a method of measuring the thermal conductivity of a substance to be measured by sandwiching the substance to be measured between substrates respectively having low and high thermal conductivities is also adopted (Published Unexamined Japanese Patent Application No. 53-107382). More specifically, on the substrate having the low thermal conductivity, a heat generating means, and a temperature measuring means located near the heat generating means are arranged. A substance to be measured is sandwiched between the substrates respectively having the low and high thermal conductivities, and when heat from the heat generating means is conducted through the substance to be measured, a change in temperature near the heat generating means is detected by the temperature measuring means, thereby measuring the thermal conductivity of the substance to be measured. This measurement method has the following merit. That is, since temperatures obtained when a substance to be measured is sandwiched and not sandwiched need only be measured, the thermal conductivity of the substance to be measured can be measured within a relatively short period of time.
In this manner, methods of measuring the thermal conductivity of a substance to be measured by utilizing thermal conduction are widely known, and can be utilized in the above-mentioned thickness measurement of a substance to be measured since their application range is wide.
However, these prior arts respectively have the following problems.
(1) The film thickness measurement based on the quartz type resonance frequency method requires a stable frequency oscillator and frequency counter, and the quartz oscillator cannot be rendered compact. For this reason, it is difficult to attain miniaturization and integration of a film thickness detector. Furthermore, this measurement is easily influenced by high-frequency noise, and is complex in procedure.
(2) The film thickness measurement based on the electron impact excitation spectroscopy requires a high-sensitive light-receiving element and a thermoelectron emitter, and results in a very expensive system. In addition, since a detection unit comprises a light-emitting element, it is difficult to attain miniaturization and integration. Furthermore, a measurement method is complex in procedure.
(3) As for the amorphous semiconductor thin film having the large Seebeck coefficient, no concrete disclosure of a thickness sensor utilizing a change in thermal resistance as the subject of the present invention is available.
(4) As a practical problem, when a measurement of the thermal conductivity of a substance is utilized in a thickness measurement of a substance, objects to be measured widely vary including, e.g., constructions, gas piping systems, structures, and the like. Thus, it is difficult to meet requirements such as an inspection of the degree of progress of deterioration or corrosion of such objects, a measurement of the thickness of an asphalt, and the like.
(5) More specifically, in the method wherein the thermal conductivity detection element is brought into tight contact with a substance to be measured to measure the thermal conductivity of the substance to be measured, the density and specific heat values of the substance to be measured must be measured beforehand by some method. When the density and specific heat values of the substance to be measured cannot be obtained, the thermal conductivity of the substance to be measured cannot be measured.
(6) In this method, in the method wherein a substance to be measured is sandwiched between the substrates respectively having the high and low thermal conductivities to measure the thermal conductivity of the substance to be measured, a measurement apparatus cannot be formed on a single plane, and furthermore, samples to be measured must be prepared. In addition, since the substance to be measured must be sandwiched upon measurement, the measurement is not easy.
(7) When the thickness of a substrate to be used is small or when a substance to be measured having a low thermal conductivity is to be measured, since the dimensions of the thermal conductivity detection element are large, and the thermal resistance is inevitably increased, sensitivity is poor, and measurement precision is limited. In addition, since the thermal conductivity element has a low response speed, the measurement time is prolonged.
(8) In order to measure a liquid concentration, e.g., an alcohol concentration in water, a method utilizing a change in refractive index is used. However, this method can only measure a concentration up to 30%. In addition, in a method using a platinum wire as a bolometer, the apparatus becomes too large.